It is known that oil is extracted from wells drilled in oil fields which are to be found in numerous types of ground and subsoils that do not all have the same production quality. This quality depends essentially on the quantity of oil that is not only available but also recoverable, and therefore it depends particularly, but not exclusively, on the permeability of the rock formation in which it is imprisoned, i.e. the ease with which the oil can flow to the well for subsequent extraction therefrom by techniques that are well known in themselves.
It will therefore be understood that it is important to know or to evaluate the permeability of any geological formation in which a fluid deposit might be worked, e.g. a deposit of oil or of natural gas.
One of the first known methods of evaluating the permeability of a rock formation consists in extracting cores of the formation and then in analyzing the cores in a laboratory. That method presents numerous drawbacks, in particular the fact that it is very difficult to deduce the real value of the permeability of the rock formation on the basis of measurements performed in a laboratory.
To mitigate that drawback, attempts have been made to implement methods and to provide apparatuses intended to measure the permeability of a rock formation in situ. The document constituted by U.S. Pat. No. 4,427,944 discloses a method and apparatus enabling the permeability of a rock formation to be evaluated in situ, e.g. down an oil well. The method described in that document consists in position an excitation source in contact with the side wall of the well and substantially at a point situated at the level where the permeability is to be measured, in controlling said source so as to cause it to produce a transient electrokinetic potential in the rock formation around the point of contact, in measuring the amplitude of said electrokinetic potential, in providing a characteristic signal that is a function of the response time of said electrokinetic potential created in the rock formation, and in determining the permeability of said rock formation around the point of contact as a function of the characteristic signal.
The apparatus that enables that method to be implemented is essentially constituted by a tool capable of setting up a transient electro-kinetic potential in the rock formation, means for positioning the tool in contact with the wall of the well substantially at a point level with the rock formation where it is desired to measure permeability, means for controlling the tool to cause it to produce excitation capable of actually delivering the transient electrokinetic potential, at least two electrodes which are pressed against the wall of the well on either side of the excitation point to measure the electrokinetic potential, and means for processing the signal delivered by said electrodes in order to determine the permeability of the rock formation at the excitation point.
That method and apparatus have the advantage of enabling the permeability of geological formations to be measured in situ in a well formed through such geological formations, however they nevertheless present the following drawbacks. The electro-kinetic potential to be detected is always very small, whereas it is Generally embedded in spontaneous potential background noise that is quite large. Contact between the electrodes and the wall of the well must be constant and excellent, and that is very difficult to achieve given the shape of a borehole, and the presence of mud in the well can set up interfering impedances between the electrodes and the wall. Consequently, even when theoretically possible, the measurement of electro-kinetic potential is not reliable and the results osf tests as provided by the means for processing the signals delivered by the electrodes are falsified.